40D ISO and Highlight tone priority

On 40D, When I turn on highlight tone priority, I can't set ISO to 100. The lowest ISO is 200.
Is this supposed to be right? Why does it work this way?

Yeah that's right but I can't explain to you why it works this way. Perhaps someone else can fill you in on this.

It limits the ISO you set so it can use the extra range to provide a safety shift and of that one stop to preserve highlights.

It is functioning as it should - read the manual.

When you set the camera to ISO 200 in HTP, the camera is set to ISO 100 EC-1 actually. Then after taking the shot, a different curve is applied than usual, wich leaves the highlights with regular 1x amplification and reach 2x towards the mids and shadows.
And since the exposure is attributed normally for the mids, It's safe to say you've shot at 2x ISO 100 = ISO 200.

Like adas said. HTP cuts the ISO in half without telling you. Since 100 is the lowest the camera can do, it can't be halved and, therefore, it can't be HTPed.

What they said Bottom line is its normal!

and its only use seems to benefit JPEG and not RAW.

jlvpeng,
Take a look at page 167 of your 40D manual and it explains it

thanks for all these comments..